Applications

Engineering

General Equipment/Test Overview

TDR—Time Domain Reflectometry Uses a fast rise time pulse launched into test sample. Reflections from impedance discontinuities are displayed along trace, displaced in time (X-axis). Impedance is not displayed directly, but can be calculated from reflection coefficient (Y-axis).

Resolution depends on how close the test sample's impedance is in comparison to the TDR's reference line impedance.

Impedance variance with length displayed creates difficulty in selecting a single measurement value.

\[Z = {R_t}\frac{{(1 + \rho )}}{{(1 - \rho )}}\]

Where:

Zo is Characteristic Impedance of device under test (ohms)

Rt is Reference Line Impedance (ohms)

r is Reflection Coefficient (Rho)

Note:—Open Circuit is defined as +1.0 r (ohms)—Short Circuit is defined as -1.0 r (ohms)—Perfect match to reference line is defined as 0r (usually 50 ohms)

C&M utilizes the Tektronix 11801A/SD-24 Digital Sampling Oscilloscope and TDR which facilitates measurement of common mode or differential Characteristic Impedance, Propagation Delay and Skew, Crosstalk, Rise Time Degradation, and length. This TDR has the ability to select part of a trace (window) and give statistical data with this trace, such as the minimum, maximum and mean impedance of a given length of cable. C&M also uses TEK 7854 and 1502 TDR's.

Network Analyzer—Resonant MethodUses measured values of Resonant Frequency and Capacitance to calculate impedance. This method is only valid for insulation materials whose electrical properties are stable over the entire applicable frequency range. The capacitance is normally measured at a low frequency range. The capacitance is normally measured at a low frequency (1kHz-1MHz) and the sample length is generally selected such that the resultant resonant frequency will be within the test samples steady-state impedance region. The test sample is typically en (10) feet, yielding a resonant frequency between 30MHz-45MHz.

Advantages

Good repeatability.

Method yields average impedance over length of sample.

Resonant frequency can be used to calculate Velocity of Propagation/Time Delay simultaneously.

Disadvantages

Time consuming due to necessity of removing fixture errors and mutual capacitance measurement.

\[{Z_0} = \frac{{1016.7}}{{Cm*{V_p}}}\]

Where:

Cm is mutual capacitance (pF/foot)

Vp is Velocity of Propagation (%100)

Network Analyzer—Open/Short MethodA cable's Characteristic Impedance is the geometric mean of the open-circuit and short-circuit impedances, measured on a Vector Network Analyzer. This method allows the impedance to be measured at a given frequency or range of frequencies which may or may not be within the cable's steady-state Impedance region. Care must be taken in selecting length of sample for test to prevent phase-related errors.

Advantages

Allows impedance to be measured at a specified frequency or range of frequencies.

Method Yields average impedance over length of sample.

Other characteristics can be derived simultaneously: Attenuation Constant, Phase Constant, and Phase Angle.

Disadvantages

Care must be taken in selecting length of sample for test to prevent phase-related errors.

Time consuming fixture-error removal (calibration) must be performed.

Network Analyzer—Insertion Loss MethodA method of measuring the attenuation of a sample at a given frequency or range of frequencies. Two ports of the analyzer are connected together and the loss is measured (dB system), then the sample is inserted between the two ports and the loss is remeasured (dB system plus sample). The attenuation of the sample is the difference between the two measurements. This method is also used for Crosstalk measurements.

Advantages:

Fixture errors are removed during calibration.

Closely resembles end-users application.

Disadvantages:

Sample length should be such that the loss measured is great enough to be above analyzer's noise floor (generally 2 dB).

Impedance matching devices, matched to sample's characteristic impedance, must be used to prevent reflections from affecting measurement.